Ionic conductivity is usually associated with a flow of ions through an aqueous solution of metallic salts. In the vast majority of practical uses of ionic conductors, i.e., as electrolytes for dry cell batteries, the aqueous solution is immobilized in a paste or gelled matrix to overcome the difficulties associated with handling and packaging a liquid. However, even after immobilization, the system is still subject to possible leakage, has a limited shelf life due to drying out or crystallization of the salts and is suitable for use only within a limited temperature range corresponding to the liquid range of the electrolyte. In addition, the necessity of including a large volume of immobilizing material has hindered the aims of miniaturization.
In attempting to overcome the shortcomings of liquid systems, investigators have surveyed a large number of solid compounds hoping to find compounds which are solid at room temperature and have specific conductances approaching those exhibited by the commonly used liquid systems. Most solid compounds that might be considered as solid electrolytes have specific conductances at room temperature (20.degree. C.) in the range of 10.sup.-6 to 10.sup.-15 ohm.sup.-1 cm.sup.-1 as compared to aqueous solutions of salts which nominally have a specific conductance of 0.5 to 0.8ohm.sup.-1 cm.sup.-1.
In addition to the above shortcomings of liquid systems, improved microelectronic circuit designs have generally decreased the current requirements for electronic devices which in turn have enhanced the applicability of solid electrolyte power sources which usually can only deliver currents in the microampere range. These solid electrolyte systems have the inherent advantage of being free of electrolyte leakage and internal gassing problems due to the absence of a liquid phase and consequent corrosion phenomena. In addition, they also have a much longer shelf life than the conventional liquid power sources.
Solid electrolytes must allow ionic transport in the crystal lattice for the cell to operate, while at the same time they must be essentially electronic insulators so as not to internally short the cell. It has been discovered that certain metallic salts which are solids at room temperatures have specific conductances sufficiently high to permit their use in practical battery applications. For example, U.S. Pat. No. 3,723,185 discloses solid state electrolytes of compounds conforming to the general formula AgI-MCN-AgCN or modifications thereof wherein M is potassium, rubidium, cesium or mixtures thereof.
U.S. Pat. No. 4,186,248 discloses a solid state electrolyte comprising an ionically conductive product of a fused ternary mixture of lead fluoride (PbF.sub.2), an alkali salt selected from the group of sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs) in a range of between about 1.0 and about 15.0 mole percent and a fluoride, nitrate or sulfate of magnesium, calcium, strontium or barium in a range between about 1 and about 40 mole percent.
It is an object of the present invention to provide a novel ionically conductive product that is solid at room temperature and has a high ionic conductance to permit its use as a solid state electrolyte for battery applications.
Another object of the present invention is to provide a novel solid state electrolyte for battery applications that is essentially an electronic insulator so as not to internally short the cell in which it is used, while at the same time it allows ionic transport in the crystal lattice.
Another object of the present invention is to provide a novel solid state electrolyte composition comprising cerium trifluoride or lanthanum trifluoride, an alkaline earth metal fluoride, chloride or carbonate, and an alkali metal fluoride, sulfate, chloride or carbonate.
The foregoing and additional objects will become more fully apparent from the following description.